Silylenes single bond insertion reactions

REACTIONS OF SILYLENES 4.1. Insertion into Single Bonds... 

Insertion reactions of silylene into a number of single bonds have been observed. The bonds include Si—O, Si N, Si H, Si halogen, strained C O, Si Si, O—H, N—H, and C—H (intramolecular only). Insertion into an X H bond can be initiated by the formation of silaylide (50) with the donation of a pair of electrons from a heteroatom X to form a bond to a divalent sihcon atom (Scheme 14.26). 

Insertion of a silylene into a P-P single bond was observed for the reaction of silylene 85 with the diphosphaetidin complex 163. The tungsten complex of l,2,4,3-azadiphosphasilol-5-ene, 164, was obtained in high isolated yields (Equation 14) <2005CC4842>. 

As represented in equation (78), the insertion of a silylene, SiRR, into an X-Y bond gives SiRR XY as a product. The insertion of silylenes into single bonds is the most well-established type of reactions for these divalent species. It is the only silylene process which leads directly to stable products in a single step. On the other hand, the immediate reaction products of either polymerization or addition reactions of silylenes are supposed to be intermediates, which may either undergo further reactive encounters or require subsequent rearrangements to give the final stable products. 

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes. ... 

As in the case of silanone 9, the reaction of the silylene bis[2-(dimethylamino-methyl) phenyl]silanediyl (8) with phenyl isothiocyanate was examined.29 In this reaction the expected silanethione 36 was obtained as a single product even in the presence of (Me2SiO)3, no insertion product of 36 into a Si-O bond of (Me2SiO)3 was observed (Scheme 10). 

Stepwise addition of two pyridine molecules to Ph-Si , whose reversibility was established in collision-induced dissociation (CID) experiments, seems to be due to the formation of one bond at a time, the monovalent silicon cation reacting as a Lewis acid. That two, but no more than two, pyridine molecules are accepted by Ph-Si points to the silicon atom as the site of addition. In this scenario, addition of the first pyridine forms a distonic silylene. That this is a plausible process is indicated by the reaction of the parent silanetriyl cation H-Si with diethylamine HNEt2 CID of the product ion established its structure as a four-membered ring whose most likely source is a two-step process formation of a silylene intermediate by a Lewis acid-Lewis base reaction followed by intramolecular insertion of the silylene into a methyl C-H bond. Three bonds are formed in a single reactive encounter, but the stepwise process is much more likely than the more interesting concerted reaction. 

---